1. Field of the Invention
The present invention relates to an optical head access controlling apparatus of an optical type information recording/reproducing apparatus for recording information in a track on an information recording medium or reproducing the recorded information using an optical head.
2. Description of the Related Art
Generally, a track is concentrically-circularly or spirally provided in an information optical medium, such as an optical disk. Information is recorded and/or reproduced by making the optical head follow the track. When information is continuously recorded/reproduced in a track, an optical head can always be made to follow a track by performing proper tracking control. However, when random access is carried out, a seeking operation for making an optical head move on a track detached from the optical head is needed. The seeking operation is performed, for example, by an optical head access controlling apparatus described as follows.
FIG. 4 is a block diagram of an optical head access controlling apparatus which has been generally used.
An optical head access controlling apparatus 50 currently in use converges optical beams 52 emitted from a light source (not illustrated) on a track of an optical disk 51 in which a track for recording/reproducing information is provided by means of an objective lens 53 as shown in FIG. 4. The light reflected by the optical disk S1 enters a bisected light receiving device 54. The light source, objective lens 53 and bisected light receiving device 54 are integrated as an optical head 55. The optical head 55 can be moved in the radial direction of the optical disk 51 by means of an optical head actuator 56. Each output of the bisected light receiving device 54 is supplied to a subtracting amplifier 57 and a track error signal 58 is output. The track error signal 58 is supplied to a pulse generating circuit 59 and pulses corresponding to zero crossing are generated. The pulses originated from the pulse generating circuit 59 are supplied to a speed detecting circuit 60. The speed detecting circuit 60 detects a moving speed of an optical spot irradiated onto the optical disk 51 from an input pulse cycle.
Further, the pulse originated from the pulse generating circuit 59 are supplied to a track counter 61. When an access operation starts, the track counter 61 subtracts the number of input pulses from a crossing track number N set by a CPU 62 controlling the whole apparatus and outputs the remaining number of tracks to a target track. The output of the track counter 61 is fed to a reference speed producing circuit 63. The reference speed producing circuit 63 outputs a speed signal in which a speed is decelerated by constant acceleration as the optical spot is approaching the target track. A speed direction designating circuit 64 and reference speed direction designating circuit 65 are switching circuits which switch between reversing and outputting the polarity of the speed detecting signal and reference speed signal which are input, and outputting them as they are. The outputs of the speed direction designating circuit 64 and reference speed direction designating circuit 65 are supplied to an error detecting circuit 66, so that a speed error signal is obtained. The speed error signal is supplied to an amplifier 67 and is amplified, and then, the speed error signal is supplied to the optical head actuator 56. By the aforesaid structure, the access control which makes the optical head 55 move to the target track is performed.
The optical disk access controlling apparatus 50 measures a pulse generating cycle which corresponds to the zero crossing of the track error signal and converts the cycle to speed for detecting the speed at accessing. Therefore, a track pitch for recording/reproducing information previously formed in the optical disk 51 is used as a basis. Further, for generating a reference speed, it is necessary to generate speed signals previously set in accordance with the distance to the target track. However, the distance in this case is obtained by the number of crossing tracks. Accordingly, the track pitch is also used as a basis. Since a track pitch is correctly produced in an ordinary disk, it is not a problem to use a track pitch as a basis for detecting the speed or distance.
However, in order to increase an information recording capacity of an optical disk, a disk having a smaller track pitch than usual has been recently used. In this case, it is desirable that an optical disk apparatus can record/reproduce information for a disk having a wider track pitch than a conventional one in the same manner as the disk having a narrow track pitch.
As mentioned above, a speed detecting circuit or reference speed producing circuit of a conventional access controlling apparatus uses a conventional track pitch as a basis. Therefore, there is a shortcoming in which a disk having a different track pitch cannot be stably access-controlled.
Although the aforesaid shortcoming can be solved if the number of the kinds of the speed detecting circuit and reference speed producing circuit which is the same number of the track pitches of an optical disk are prepared and the speed detecting circuit and reference speed producing circuit are switched with each other, there is a problem in that the size of the circuit must be enlarged.